This invention relates in general to color television cathode ray tubes. Conventionally, a color television cathode ray tube has a glass bulb which is fabricated in two parts; a funnel and a flanged faceplate which is sealed to a flared end of the funnel. Conventional color cathode ray tube faceplates typically comprise a dished viewing portion having a concave inner surface upon which a phosphor screen is deposited. The screen comprises a mosaic of intercalated patterns of red-emissive, blue-emissive and green-emissive phosphor elements. The patterns of elements are deposited in succession, each by a series of operations which includes the application of a coating of an aqueous phosphor slurry to the faceplate inner surface. This invention concerns an improved process for applying such phosphor slurry coatings to the inner viewing surface of a color cathode ray tube faceplate. The phosphor slurries involved typically include a photosensitized organic binder and a suspended particulate phosphor material having a predetermined particle size distribution. The organic binder, typically PVA (polyvinyl alcohol), its sensitizer, and the phosphor material are commonly collectively termed the slurry "solids."
All known methods for disposing phosphor slurry coatings on color cathode ray tube faceplates involve an operation wherein a quantity of phosphor slurry is suffused (spread) across the faceplate surface to be coated. This suffusion operation generally involves the application of a puddle of slurry to the surface to be coated, followed by one or more operations which cause the puddle of slurry to be spread across all areas of the surface. The excess slurry is then removed. Finally, a "levigation" operation is employed by which the suffused coating is leveled and thinned down to a predetermined thickness; typically this is accomplished by very rapidly spinning the faceplate, or in the disclosure of U.S. Pat. No. 3,700,444, by inverting the faceplate.
A number of prior art methods for applying phosphor slurry coatings have been developed. The commercially most common methods involve applying a puddle of slurry onto the concave inner surface of a faceplate while the faceplate is in a face-down position. The puddle is then suffused across the surface to be coated by tilting and rotating the panel to cause the puddle to move across all areas of the entire surface to be coated, or alternatively, by spinning the panel to cause the puddle to spread by centrifugal force across the faceplate inner surface. For example, see RCA Review, Vol. 16, pp. 122-139 (March, 1955) and U.S. Pat. Nos. 2,902,973; 3,319,759; 3,376,153; 3,364,054; 3,467,059; and 3,700,444.
Conventional faceplates have a rearwardly extending flange which contains the puddle. This invention is applicable to such conventional faceplates and color cathode ray tube faceplates in general, but is perhaps especially suited for use with a color cathode ray tube faceplate having a dished viewing portion but no flange. The tilt-type process is not applicable to a flangeless faceplate, since such a faceplate has no flange to retain the slurry on the faceplate during the tilting and rotation of the faceplate. Without the flange the slurry would flow over the edge of the faceplate before the coating operation is completed.
The afore-described prior art methods, in general, involve a coating leveling or levigation process in which the faceplate is spun at high speeds to thin down and level the coatings of phosphor slurry which have been suffused across the faceplate inner surface. By the nature of the puddling-type processes, a relatively thick layer of phosphor slurry is deposited upon the faceplate surface. In order to thin down this thick layer to an acceptable coating thickness, the panel is spun at a high speed (e.g., 250-300 RPM) for a relatively long interval, e.g., 10-15 seconds.
It is known in the commercial manufacture of black grilles for color cathode ray tubes of the black matrix type to apply a uniform layer of a graphite material to the inner surface of a color cathode ray tube faceplate by flowing a graphite solution onto the faceplate while it is being rotated in a substantially vertical attitude. Such a process is disclosed in U.S. Pat. No. 3,652,323. A similar process is used earlier in the grill-making operation to apply a photosensitized PVA coating under the graphite layer. The graphite and PVA coating processes have little or no relevance, however, to the invention described and claimed herein.
The reference copending application Ser. No. 592,431 discloses a non-settling process for forming on an inner viewing surface of a color cathode ray tube faceplate, a coating of an aqueous slurry composed of an organic binder and a suspension of particulate phosphor material of distributed particle size. The process comprises supporting the faceplate such that the axis of the faceplate has a substantial horizontal component, and slowly rotating the faceplate about the central axis thereof while dispensing a stream of phosphor slurry having a predetermined phosphor particle size distribution onto the central region of the faceplate inner surface, preferably, substantially at the axis of rotation. Due to gravitational forces and the slow rotation of the faceplate through the descending slurry stream, the slurry is suffused to the perimeter of the faceplate inner surface without any significant settling out of the phosphor particles onto the faceplate. The coating is then levigated by rotating the coated faceplate at a moderate angular velocity for a brief time interval, the joint effect of which moderate angular velocity and brief time interval being to level the coating down to a predetermined thickness while suppressing the formation of radial streaks in the coating.
That process has been found effective to apply a uniform slurry coating, however, in spite of its relatively high through-put rate, even higher rates are desirable.